Modeling of Transport Mechanisms of OH- in Electrolyte of Alkaline Fuel Cell

In recent years, interest has grown in the development of anion exchange membranes (AEMs) for alkaline fuel cells, which have advantages over PEM fuel cells including cost and performance. In a basic environment using AEMs, sluggish oxygen reduction reaction at the cathode can be significantly improved, leading to high fuel cell efficiency [1]. Catalysts are generally more stable in basic medium so that more options are available for the cathode catalysts from nonprecious metals (such as nickel and silver) and thus potentially reducing the cost of the fuel cells. However, problem exist the degradation by hydrolysis with a base, and development of the electrolyte film with high conductivity. Therefore, in this study, we reveal the electrolyte polymer which have the high conductivity by the molecular dynamics and compare two kinds of polymers and clarify structure to express the high conductivity (diffusibility). As a result we have found that coefficient of OH- increases as increase in the number of the sites of CH₂^-N ⁺ (CH₃)₃, and the diffusivity in QPE (quaternized polyethers) is 2.538×10 ^-9 m2/s and that in PPESK(poly (phthalazinone ether sulfone keton)) is 8.579×10 ^-10 m2/s. We investigated that which is working predominantly, Grotthuss mechanism and Vehicle mechanism are thought as diffusion mechanism of OH^�|. We will reveal whether there is regularity in hydrogen bonding and movement of the molecules of the Grotthuss mechanism. Grotthuss mechanism has more diffusion than Vehicle mechanism, because Vehicle mechanism is the diffusion by the OH^�|moves between sites CH₂^-N ⁺ (CH₃) ₃ and depends on the number of sites, but a lot of water molecules keep moving in the Grotthuss mechanism [2] . In addition, by the Grotthuss mechanism of OH^�|, in the case of molecular movement, OH^�|moved along conjugate of the hydrogen bonding and was able to confirm a regularity to form H₃O₂ and H₅O₃⁺. This kind of diffusion behavior of OH^�|is completely different with that of H⁺. Furthermore, the degradation mechanism is also investigated using the data of molecular dynamics and quantum chemistry calculation for better understanding of the degradation of polymers by OH^�|.

Reference:

1. M. Tanaka, M. Koike, K. Miyatake, M. Watanabe, Macromolecules, 43, 2657 (2010)

2. G. Merle, M.Wessling, K. Nijmeijer, J. Membrane Science 377, 1 (2011)